There is strong evidence for the induction of the inflammatory enzymes 5-lipoxygenase (5-LOX) and cyclooxygenase-2 (COX-2) in atherosclerosis, asthma, and many types of cancer. Both enzymes mediate the inflammatory response of these diseases through the synthesis of leukotrienes and prostaglandins, respectively. The leukotriene and prostaglandin pathways have traditionally been viewed as independent biosynthetic routes since the committed step toward either pathway is taken as the initial oxygenation of the common substrate, arachidonic acid. This proposal is based on the finding that the 5-LOX product, 5S- hydroxyeicosatetraenoic acid (5S-HETE), is a selective and highly efficient substrate for oxygenation by COX-2 (but not by the COX-1 isozyme). The discovery of this novel substrate for COX-2 intimately links the two eicosanoid pathways. The common 5-LOX/COX-2 product is a novel di-endoperoxide that is structurally reminiscent of, but distinctly different from prostaglandin H2. In the first Specific Aim we propose to analyze the enzymology of the conversion of arachidonic acid into the novel di-endoperoxide by establishing the reaction kinetics, substrate specificity, and the structural basis for binding of 5-HETE in the COX-2 active site. The enzymatic transformation of the di-endoperoxide product will be studied using RAW264.7 cells and murine peritoneal macrophages, prototypical cells that expresses both 5-LOX and COX-2. Endogenous formation of the novel eicosanoid and its family of metabolites will be assessed in mouse atherosclerotic tissue. We will test the hypothesis that the novel di-endoperoxide (or its metabolites) possess anti- inflammatory properties. Preliminary studies have shown that the di-endoperoxide attenuates the release of interleukin-8 from stimulated mast cells and microvascular endothelial cells. The signaling pathways involved in this interaction will be studied using the 5-LOX and COX-2 expressing HMC-1 human mast cell. Characterization of the novel 5-LOX/COX-2 cross-over pathway will undoubtedly lead to an entirely new understanding of the biology of 5-LOX and COX-2, and it will also shed new light on the etiology and regulation of the inflammatory component of diseases like atherosclerosis, asthma, and cancer. These studies could ultimately lead to new therapeutic regimens for the treatment of these diseases using established anti-inflammatory medications.